Untrapped Box Gulley 3D Detail Drawing, Installation and Item Cost Rate Build-up

Fig 1 – Detailed 3D Model

Brief Details

Polypropylene (PP) untrapped rainwater gulley.

Gulley with sloped quadrilateral box, vertical and horizontal outlets.


  • Overall size: 300 x 166 x 215mm high.
  • Box Top:  300x166mm wide.
  • Box Height: 75mm on smaller end and 160mm on higher end.
  • Overall height of gulley including vertical outlet: 195 to 215mm high
  • Diameter of vertical and horizontal outlets:  100mm internal dia. and 110mm external dia.
  • Top inlet for round rainwater pipe: for 50,75,80,90,100,110 and 125mm diameter pipes.
  • With removable internal baffle plate.
  • Pre-fitted with a removable blanking plug to close unused drain outlets. The plug can be removed to allow rodding and connecting to a drain pipe.
  • 120mm diameter guard grill for untrapped rainwater gulley to keep out leaves. Placed on vertical and horizontal outlets.
  • Gulley installed on concrete slab base at least 75mm thick.

Fig 2 – Detailed 3D Model

What is an Untrapped Rainwater Gulley?

An untrapped gulley is made in a way that doesn’t trap water at the bottom. Water flows through the catchbox without a part of it being trapped at the bottom. To ensure that no water is trapped, the drain outlet is positioned at the bottom of the catchbox and alternatively if the drain outlet is to be positioned on the sides or front, it is fixed flush with the bottom level of the catchbox and the gulley bottom is sloped at a suitable angle to allow a much faster rate of water flow. If a gulley doesn’t have an inbuilt trap or if it’s not connected to a trapped drain pipe, a leaf guard grill is usually placed on the drain outlet to keep out leaves and litter which may clog the drain pipe.

The function of a rainwater gulley is to collect water from rainwater downpipes, surface water runoff or both. There are different types of gullies made of this purpose. A gulley that is made for the purpose of taking water from rainwater downpipes only has a top inlet that allows a downpipe to be connected through. This type of gulley will not collect surface water runoff, therefore it’s not suitable for surface water drainage.

The gulley shown in this post is a box-shaped type, untrapped and useful only for taking water from rainwater downpipes. It has a top inlet for connecting an 80mm diameter downpipe. Depending on the manufacturer’s model, some gulleys, for example the types made of Polypropylene (PP) are made in a way that allows the builder to select a custom downpipe diameter on the top inlet by removing a pre-pressed ring. Thus you can choose to make a custom inlet by removing a 50,75,80,90 and 100mm diameter ring respectively.

The box for this gulley is a quadrilateral shape measuring 300x166mm wide and 215mm high overall. The box comes with a horizontal and vertical outlet of 100mm internal diameter, which allows you to fit a 100mm diameter PVC drainage pipe. Each of these outlets can be closed with an internal removable baffle plate if they are not needed for connecting to a drainage pipe. If you are using a horizontal outlet to connect a drain pipe, then you have to close the bottom vertical outlet with a baffle plate. The vice versa applies to the vertical outlet. If you are using the vertical outlet to connect a drain pipe, then you have to close the horizontal outlet.

This untrapped box gulley is the simplest gulley that you can choose among various types.

Fig 3 – Detailed 3D Model

How to Install a Rainwater Box Gulley:

Provided that the rainwater downpipes are already in place, fixed to the walls complete with the roof gutter system, you can start digging a hole under the downpipe. Installing the gulley after the downpipes are completely installed makes it easy to position the top inlet of the gulley in line with the vertical centreline of the downpipe.

Dig a hole that allows the box gulley and bottom concrete base to fit in. The top of the gulley is going to be flush with the ground level, paving or concrete apron. The overall dimensions of the gulley including the bottom outlet are 300x166mm wide x 215mm high. It will rest on a concrete base approximately 500mm x 250mm wide x 75mm thick.

The overall height including the concrete base is [215 + 75 = 290mm]. This is going to be the depth of the hole excavation. Provide an allowance for a 50mm sand bed or blinding. This makes it 290 + 50 = 340mm deep. Therefore, excavate a hole 500 x 250 x 340mm deep, and store excavated material on site. If you are going to use the excavated material for compacted back-filling to the bottom and sides, make sure the soil is free from organic matter, leaves, grass, twigs and rubbish.

Fig 4 – Detailed 3D Model

Excavate Trench for 100mm PVC Surface Water Drainage Pipes:

After digging the hole, you have to excavate a trench for laying PVC drainage pipes that will be connecting to the gulley. If the drain pipes are not yet delivered on site, you can simply extend the length of the hole (if it’s not long enough) by digging a short trench about 500mm long to allow for future working space when the drain pipes are actually installed on site.

Compact the bottom of the excavation to 95% Mod AASHTO density with a tamper or vibrator. Spread a layer of sand 50mm thick, and start preparing concrete. Unreinforced concrete [class 10 to 15MPa/19mm] will be suitable under excavated surfaces. Pour in wet concrete about 75mm thick, vibrate and tamp the concrete to eliminate air bubbles. Leave the concrete to set and cure.

When the concrete is dry and cured, it’s time to install your box gulley. Place the gulley inside the hole on the concrete base and connect the top inlet to the downpipe. Back-fill the sides with earth-filling.

Fig 5 – Detailed 3D Model

Bill of Quantities Item Cost Rate Build-up for Rainwater Gulley:


Untrapped PP gulley [Brett Martin] Prime Cost: US$22.68

Add Labour: US$6.80

Sub Total: US$29.48

Excavation:         US$2.79

Compaction:      US$0.09

Concrete base 500x250x75mm: US$0.22

TOTAL:  US$32.58

Fig 6 – Detailed 3D Model